One relatively simple and painless procedure for saving on the costs of heating or cooling a space is to change the set point temperature for the thermostat in it when the space is not occupied, or in the case of a home at night when the occupants are sleeping. However, since it is not physically possible to warm up or cool down the controlled space instantaneously, to maintain comfort for the occupants it is necessary for the space temperature control unit (furnace or air conditioner usually) to begin changing space temperature prior to the time when occupancy is scheduled again to occur. We will hereafter refer to the time of day when temperature in a space is to reach a higher energy use level (comfort level) as an occupancy time and the temperature selected for that interval of occupancy as the occupancy temperature. Note that this definition also includes the higher temperature which is typically selected during a heating phase of space temperature control and which follows a nighttime setback interval even though, strictly speaking the space has never been unoccupied. The interval during which the space temperature returns to the occupancy temperature from a temperature which requires less energy to maintain is referred to as the recovery interval, and the time of day at which the recovery interval begins is the recovery start time. The time at which the space temperature reaches the occupancy temperature is the actual recovery time. Since there is typically a temperature range centered on the occupancy temperature, or any other set point temperature for that matter, actual recovery time will be considered in the following discussion to have been reached when the space temperature first enters this temperature range.
The length of the recovery interval depends on the thermal load to which the space is subjected for two different and distinct reasons. If in heating mode for example, lower ambient temperature will require a longer recovery interval, i.e. the space will take longer to warm to the occupancy temperature, because the thermal load on the space is high. If the ambient temperature is low and the setback period is long, the space temperature will reach the setback temperature, further increasing the recovery time. On the other hand, a higher ambient temperature may provide a sufficiently reduced thermal load that the space does not reach its setback temperature and hence the difference between the space temperature at the time the recovery interval begins and the occupancy temperature is small, and the space temperature increases rapidly as well because there is a smaller thermal load for the furnace to overcome. The analysis is similar for air conditioning setback to a higher temperature and then a subsequent recovery interval to the lower occupancy temperature. The invention to be described is equally applicable to heating and to cooling, but for convenience, the disclosure will mainly address a heating situation and this should be assumed unless otherwise stated.
Typically, the programming of the setback and recovery times and temperatures is done to a thermostat physically located in the controlled space. The most modern of these thermostats include a small microprocessor and a suitable display and control panel allowing an internal clock to be set and the desired time and temperature for each particular time period to be entered by the manager of the space. The thermostat to be described is controlled by the operation of a microprocessor type of digital processor.
It is desirable to start each recovery interval at a time which will place space temperature at the occupancy temperature very close to the occupancy time. After all, what is the purpose of accurately selecting setback and occupancy times if the associated occupancy temperatures are not achieved with reasonable accuracy? But this is not easy to do accurately, because there are a number of independent variables which ar difficult to take into consideration when designing an inexpensive thermostat. For example, the rate at which individual heating units can heat a space is variable. As mentioned, the ambient air temperature, solar radiation, and wind can vary dramatically from one day to the next, further increasing the uncertainty about the thermal load on the space against which the heating unit must work. It is possible to include sensors which can to some extent measure these variations, but this contradicts the concept of a low cost thermostat which is easy to install.
Accordingly, it would be advantageous to design a thermostat having a recovery algorithm which is relatively sensitive to factors affecting recovery time and yet does not require expensive sensors to measure these factors. One solution used in the past is to record recent recovery times, and base the current prediction for the recovery time needed on recent recovery times. U.S. Pat. No. 4,522,336 discloses one means of implementing this concept. This patent teaches measuring the time for the space temperature to traverse some temperature range while the space temperature control unit is operating to determine a rate of temperature change, and then using this rate of change with an adjustment factor to determine the proper recovery start time.